The present invention relates to the field of optical fiber communications, and more in particular, to an optical channel cross connect for communication systems in WDM (Wavelength Division Multiplexing) technique, having a double spatial switching structure on optical flows, strictly not blocking, and interposed functional units operating on each single flow.
Starting from the first appearance of the optical fiber as physical carrier in telecommunication networks, the domain of the technique, the present invention falls under, has been characterized by an ever-increasing progress in optical devices, which enable such form of communication. We can briefly mention the following optical devices available on the market at the date:
Erbium Doped Fiber Amplifiers, known with the, acronym EDFA, which, pumped by a laser signal at an appropriate wavelength xcex, can amplify WDM optical flows having total capacity highly exceeding 10 Gbit/s, maintaining a sufficiently flat gain response within a band of minimum attenuation of the single-mode optical fiber, typically ranging from 1530 to 1565 nm.
Band-pass filters having bandwidth lower than 1 nm, capable of being electronically tuned from one to any other wavelength, inside the above mentioned spectral interval of approximately 35 nm, within switching times of some, having low insertion losses, and low crosstalk among different channels (lower than xe2x88x9230 dB).
2xc3x972 switching elements in waveguide on Lithium Niobate substrate, material capable to remarkably change its refraction index under the action of a relatively strong electrical field. These devices are used to implement optical path selectors which can be used as the basic elements of a Mxc3x97N spatial division switching matrix, that can be obtained in a unique integrated circuit of the PLC type (Planar Lightwave Circuit). According to the present technology, it is not convenient to integrate matrices having dimensions larger than 8xc3x978; the realization of more expanded optical matrices requires the assembly of several PLC devices duly interconnected through optical fibers.
Electro-mechanically controlled optical selectors, capable to spatially switch one of the N input optical flows to the unique outputs; these elements can be combined to build more complex matrixe structures, characterized by dimensions up to 16xc3x9716, at the state of the art.
Semiconductor optical amplifiers, known with the acronym SOA (Semiconductor Optical Amplifiers), based on the principle of travelling wave amplification. It is possible to implement simple high-isolation optical switches by driving the active device of such an amplifier to interdiction or saturation. These components are profitably used for the implementation of several different optical devices, among which the Mxc3x97N matrices and the wavelength converters.
Broad band combiners (Optical Combiner) of N input optical flows, to form a unique output optical signal, that is the sum of the N input flows. In the case the flows entering a combiner have each one a different wavelength xcex, an output signal is obtained, consisting of the wavelength division multiplexing of the entering flows, technique known under the acronym WDM (Wavelength Division Multiplexing).
Broad band splitters (Optical Splitters) splitting on more paths a unique entering signal, obtaining a plurality of identical, though attenuated, output signals. In practice, it is possible to implement an optical splitter by simply exchanging the inputs and the outputs of an optical combiner, considering that said optical components are reciprocal.
WDM signal demultiplexers (Wavelength Demultiplexers) accepting a WDM input signal composed of N wavelengths xcex, and sending each of them to one of the different N outputs. The filtering property of these components, if realized in PLC technology, is generally obtained through a particular arrangement of planar waveguides, implementation known as AWG (Arrayed-Waveguide Grating).
Wavelength Converters that can be implemented according to different physical operation principles, for instance, by driving a SOA amplifier device to operate in a non-linear gain zone. It must be noticed just from now that the wavelength conversion functionality can be obtained also through Optical/Electrical/Optical (O/E/O) conversion, such as that made in case of optical signal Regeneration.
Optical transmitters, for the transformation of electrical signals present at the transmission interfaces of terminal stations intoloptical signals, suitable to fiber transmission. They typically include a semiconductor Laser emitting with high stability on a particular wavelength, a driving circuit imposing an on-off modulation of the light signal, acting either on the laser itself (direct modulation) or on an external optical modulator placed after the lagser (external modulation). Laser structures satisfying the requirements of a WDM system are for instance the Distributed Feedback (DFB) lasers; should the tuning capability of the laser be required on a wide spectrum interval, it is possible to consider different solutions, like the DBR (Distributed Bragg Reflector) structures.
Optical receivers, for the reverse transformation of the optical signal into the corresponding electrical signal carried, at the receiver interfaces of the terminal stations. They typically include a photodiode, made of adequately doped semiconductor material, and the electronic circuits for amplification, clock extraction, data reading.
The wide possibility of selection of optical devices can greatly facilitate the transition towards optical fiber networks, where not only the transmission of channels, but also the routing of flows among different nodes is performed within the optical layer, while in the present transport networks both the space and the time switching of channels are implemented in the electrical domain and require a double signal conversion, from optical to electrical and vice versa.
The WDM technique (acronym used in the optical sector in place of the FDM term used in the radio sector) can become therefore a key factor, not only to increase the transport capacity of the already existing optical infrastructures (enabling to transmit several channels in one fiber), but also toll increase the network flexibility, availing of the wavelength as additional degree of freedom for the switching, applying the principle of transparent optical path (xe2x80x9cWavelength Pathxe2x80x9dxe2x80x94WP). Then, if the additional function of wavelength conversion is available, the possible blocking conditions due to the non flexible assignment of wavelengths to fiber paths can be overcome: in fact, it becomes possible to route two channels, entering the node at the same wavelength, from different fibers, towards one output fiber, by converting the wavelength of one channel; this solution leads to the technique defined as Virtual Wavelength Path (VWP).
It must be pointed out that channels can be routed in a cross-connect, in the widest meaning of the term, by means of spatial switching, or wavelength-based switching, or through time demultiplexing and switching.
In spite of all its advantages, the present technology does not yet make the direct switching of digital packets convenient in the optical domain, consisting for instance of ATM (Asynchronous Transfer Mode) cells. This is due to the difficulty to fully implement in the optical domain memories and data processing devices, being nowadays still at research and development prototype level.
On the contrary, in the context of the spatial and of the wavelength-based switching, the known art proposes different solutions. The main difference between these two approaches lays in the fact that in the second instance, the wavelength conversion is absolutely necessary and the routing is made by selecting for the transmission a particular wavelength; while in the context of the spatial routing, the wavelength conversion is optional and is used not to support the switching, but to decrease the blocking probability, induced by constraints external to the node (occupation of wavelengths on the output fibers).
However, in both the cases, the full optical cross-connection is largely independent on the format and bit-rate of the original electrical flows, whether SDH, PDH, ATM, or other available format.
The present proposal will relate to a space switching optical cross connect, capable of performing wavelength conversion, where required.
A more general outlook on the scenario of existing solutions is however summarized below. It is worth to show some typical optical fiber network topologies, in order to represent the application field of the present invention, with the aid of FIGS. 1, 2, 3 and 4, mentioning the following articles which expand the concepts described up to now and offering a rather wide global picture on the problems of optical cross-connection:
xe2x80x9cIs there an Emerging Consensus on WDM Networking?xe2x80x9d, by Charles A. Brackett, published on JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 14, no. 6, June 1996.
xe2x80x9cOptical Path Cross-Connect Node Architecture for Photonic Transport Networkxe2x80x9d, by Satoru Okamoto, Atsushi Watanabe, and Ken Ichi Sato, published on JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 14, no. 6, June 1996.
xe2x80x9cDesign and Implementation of a Fully Reconfigurable All-Optical Crossconnect for High Capacity Multiwavelength Transport Networksxe2x80x9d, authors Amaury Jourdan, Francesco Masetti, Matthieu Garnot, Guy Soulage, and Michel Sotom, published on JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 14, no. 6, June 1996.
xe2x80x9cAn Optical Cross-Connect System as a HighSpeed Switching Core and its Performance Analysisxe2x80x9d, by Yongdong Jin and Mioshen Kavehrad, published on JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 14, no. 6, June 1996.
FIG. 1 of the present application shows a possible switching network scheme, based on a star-coupler architecture, called in this way due to the star topology characterizing the same. Said architecture finds wide application in point-multipoint connections, such as for instance in the distribution of TV signals in CATV (Cable Television) systems. In FIG. 1, the star-coupler placed in node A receives optical signals at different xcex on a same number of entering fibers and performs the WDM multiplexing on each outgoing fiber; users B, C, D, E, placed at the remote end of a relevant connection, are equipped with a channel filter, possibly tunable, to drop the channel of interest from the received broad band signal. According to what said above, the star-coupler of FIG. 1 performs a passive routing broadcast-and-select network; however this does not prevent the possibility to use the same star-coupler in different manner in non-broadcasting networks, of the type shown in FIGS. 2, 3 and 4, for instance causing the channel filters to be included in node A, as described also in the last paper mentioned above. Similar applications appeared in the following PCT applications published under No. WO 95/13687 and WO 97/31504. The quotations describe a relevant optical cross-connect for N input WDM signals and a same number of output ones, each flow including a set of M wavelengths that similarly repeats on the N input and output fibers. Both the described architectures include N input modules connected to a same number of output modules through N star-couplers, couplers, and foresee the possibility to perform a conversion of the wavelength set in the input stage, to avoid blocking conditions at the output, and a complementary conversion in the output stage to render the node transparent. Output modules of both the architectures are similar since they include: Mxc3x97N tunable channel filters, followed by a same number of xcex converters, and include also N optical combiners having M inputs. The difference in input modules is mainly due to the fact that in each one of the N input modules of the optical cross-connect of the first quotation (WO 95/13687) it is foreseen the use of a set of tunable filters to separate the M channels, while in the second quotation (WO 97/31504) said use is not foreseen. This is the reason for which the N star-couplers of the first quotation have M single channel inputs, while those of the second quotation have N multichannel inputs. The mentioned documents emphasize the advantages of a node architecture based on the star-couplers rather than on the more traditional and expensive spatial division matrices.
It is interesting to notice that in the description of both the optical cross-connects a similar method is described to perform the drop/insert function, which consists in equipping a given number of optoelectronic input modules for the insert function and a same number of output modules for the drop function. This strategy is rather recurrent also in network nodes employing a spatial division matrix and has the drawback of requiring an increase of the size of the matrix or star-couplers, to accommodate the additional optoelectronic modules. On the contrary, when the matrix or star-coupler size has to be kept unchanged, the cross-connector would inevitably avail of a lower number of input and output modules for the primary optical flows.
FIG. 2 shows an optical fiber ring connecting four points, or nodes, A, B, C and D, respectively, placed along the circumference. These include all the optical equipment required for the operation and maintenance of the ring, as well as for the interfacing towards networks or local terminals. The growth in the traffic demand for a transport network of this type leads to estimate in some tens of Gbit/s (short term) and some hundreds of Gbit/s (medium term) the total capacity of WDM signals that must transit across each node of the ring. The ring of FIG. 2 can substantially extend on the territory, reaching a circumference of several hundreds of kilometers, or even some thousands, with some tens of nodes along the ring. Optical signals can cross the ring either in the CW (clockwise) or in the CCW (counterclockwise) direction, according to the implementation methods shown in FIGS. 5 and 6.
The configurations assumed by the fibers to implement the ring protections, are shown in FIGS. 5a and 6a, following the conventions adopted in FIGS. 5 and 6.
FIG. 3 shows a system composed of two rings similar to the one of FIG. 2, intersecting in nodes N1 and N2, which are therefore characterized by a more complex implementation. The generalization towards several intersecting rings is consequent, but as the number of rings increases a mesh network configuration, similar to that shown in FIG. 4 can result to be more advantageous.
Referring to FIG. 5, we can notice that the bi-directional traffic, crossing the nodes of ring in FIG. 2, is supported by employing two concentric rings, a first one for the clockwise CW propagation direction and a second one for the counter-clockwise. CCW direction. Limiting the description only to the node A, a xe2x80x98west sidexe2x80x99 and an xe2x80x98east sidexe2x80x99 can be identified, on the left and on the right of the node center line, respectively. The ports of the west side of node A are respectively connected to the input fiber of Rx signals travelling on the CW ring and to the output fiber of Tx signals travelling on the CCW ring, while the ports on the east side are respectively connected to the output fiber of Tx signals travelling ion the CW ring and to the input fiber of Rx signals travelling on the CCW ring.
Making reference to FIG. 6, we can notice the bi-directional traffic, crossing the nodes of ring in FIG. 2, is supported by employing a unique fiber for the two transmission directions of the signal; it is then clear that the distinction between the signals flowing in CW or in CCW directions is obtained by assigning a different set of wavelengths to the two groups.
FIG. 5a shows a fiber configuration that assures a complete protection to the double ring of FIG. 5 through repetition of each ring, leading to a total of four rings. This configuration enables to implement protection schemes known under the term of 4F-BSHR (4-Fiber Bi-directional Self Healing Ring), that corresponds to a double directional protection. In FIG. 5a the signals concerning the different fibers are denoted as the Rx and Tx signals of FIG. 5, adding the letter W (Working) for the signals on the fibers usually in serviceand the letter P (Protection) for the signals on redundant fibers.
FIG. 6a shows a bidirectional protection that is simply obtained by the repetition of the ring and of the relevant flows of FIG. 6.
From the practical viewpoint, it is also possible to give a different representation of the signals of FIGS. 5 and 5a, where Rx signals are all shown on the left of the node, while Tx signals on the right hand. This representation, though less realistic than the original one, facilitates the schemeatic representation of a cross-connect; of course in the new representation, the association of the left and right part, of the node to an eastern and a western side is no longer meaningful.
One of the most important requirements to the manufacturers of optical communication system is the flexibility of the architecture proposed for the ring node in the network context. Flexibility must be ensured both during the normal operation and in presence of critical situations due to possible failures in fiber equipment and/or connections.
The first point implies the following requirements for the cross-connect:
it should be strictly not blocking, enabling to route any input channel, at any xcex, to any output;
the interconnection status should be dynamically reconfigured, and the broadcasting of a channel, to all the outgoing fibers, allowed;
the regeneration of all the throughput channels, at different xcex, should be possible
the drop/insert of all the channels, at different xcex, from/to all the WDM flows should be enabled, or, at least, of a fraction of them;
the conversion of any of the input channel wavelength to any of the output channel wavelength should be possible, for the whole set or at least for a sub-set of wavelengths;
the number of input/output ports and/or the number of channels per fiber can be easily increased fulfill higher traffic demands, property usually called scalability of the node.
As far as the second point is concerned, the possibility of performing the protection of the equipment located in the ring nodes, including also the node access devices in local environment, and the protection of fiber paths at single link and at ring level, should be foreseen. Moreover, customers urge the need of flexible protection schemes, which can be freely configured after the equipment installation, and, possibly, also during operation, avoiding to fixing these schemes during the network specification phase, as it happens up to now.
It is clear that the technical characteristics qualifying the capability of the system in facing the failures are not completely independent from those determining the operation flexibility. The manufacturers have a wide margin in designing the node architecture they consider the best one, but it is also true that it is rather difficult to conjugate in a particular architecture all those characteristics, often in conflict among them, suitable to simultaneously satisfy, at competitive costs, all the above mentioned requests.
The architectures seen up to now do not attain this purpose; considering in fact the optical cross-connects described in applications PCT WO 95/13687 and WO 97/31504 mentioned above, we highlighted for instanrce the incapability to implement at limited cost the drop/insert function on all the transit channel.
Another drawback, typical of the architectures implementing the spatial switching of WDM signals through star-couplers put between the input and output modules, is the necessity of using two Mxc3x97N tunable filter groups, to obtain a strictly non-blocking configuration; this can be seen for instance in the first of the two patent quotations.
Concerning the already mentioned classical architecture of optical cross-connector based on optical demultiplexers followed by a spatial switching matrix followed by optical multiplexers, we can notice that it assures valid performances only if constructed around a unique non-blocking spatial matrix, by which all the throughput channels are routed. Successful demonstrations have been made in the field of research projects, however with limited size matrices (up to 16xc3x9716). Said dimensions are insufficient, in the light of realistic traffic requirements of an optical transport network; furthermore, the need to assure the drop/insert functionality of channels tends to require an additional increase of the optical matrix size. Matrices having size 32xc3x9732, scalable to 64xc3x9764 and 128xc3x97128, seem to be the minimum requested, in the context of this architecture; these are neither available at the state of the art, nor are expected to become available in a short time, considering also the urgent transmission performances necessary in a transport optical network.
Another disadvantage of these architectures is that a flexible choice of the set of used wavelengths is not possible, being the wavelength set fixed by WDM demultiplexers and multiplexers.
Therefore it is necessary to thoroughly investigate in the background art to look for better solutions. An optical cross-connect, which at a first sight could be considered as a good candidate, since it does not em ploy large and expensive spatial matrices, is the one based on the so called xe2x80x9cparallel xcex switchxe2x80x9d architecture; this is outlined, for instance, at page 1414 of the volume containing the second paper mentioned above (Satoru Okamoto et al.), where it is also pointed out that the described architecture offers higher modularity and scalability, with respect to other structures. The xe2x80x9cparallel xcex switchxe2x80x9d architecture is clearly represented in the FIG. 7 of the present application (corresponding to FIG. 8 of the quotatio).
Making reference to FIG. 7, we can notice that the mentioned optical cross-connect includes N input optical fibers carrying a same number of WDM input signals, and N output optical fibers carrying a same numbler of WDM output signals, each WDM signal consisting of N elementary flows at different wavelengths, xcex1, . . . xcexM. Each entering fiber is connected to a relevant first broad band optical splitter, 1:N splitter, dividing the entering WDM signal in N identical WDM output signals, whose power is N times lower that that of the entering signal. Therefore, N2 output signals are on the whole available, to be connected, according to the methods defined below, to the input of a same number of second broad bland optical splitters, 1:M splitter, dividing the WDM signal at their input in M identical WDM output signals, whose power is M times lower than that of the entering flow, and therefore, Nxc3x97M times lower than that of the signal on the corresponding entering. fiber. The double level of splitting makes available in total N2xc3x97M WDM signals; Nxc3x97M times attenuated with respect to the optical cross-connect input.
The connection between the first and second splitters takes place dividing these last in N groups of N elements each, and therefore connecting each output of an n-th first splitter to a second splitter of a group having the same numeral order of the considered output and the n-th position in the relevant group.
The selection of the Nxc3x97M signals to be routed towards the N output fibers, among the N2xc3x97M WDM signals coming out from the whole of the second splitters, is done, in the cross-connect of FIG. 7, by Nxc3x97M broadband optical selectors, each one consisting of a switch Nxc3x971 having N selectable inputs and one output, connected to a tunable channel filter, Tunable Filter, that can select any of the M wavelengths xcex1, . . . xcexM of the entering signal. The Nxc3x97M optical selectors are divided in N groups of M selectors each, univocally associating the n-th group of optical selectors to the n-th group of second splitters and, for each pair of associated groups, connecting the M outputs of each second n-th splitter to the n-th inputs of the M optical selectors.
The output of the selectors are connected to a same number of Optical Regenerators, Optical Receiver, Optical Sender, regenerating the filtered signals and taking care to restore the correct power value of the signals transmitted forward. Due to the association between groups of second splitters and groups of optical selector, also the filters and the regenerators placed downstream the selectors turn out to be associated in identical manner. The outputs of M regenerators of a n-th group are sent to a same number of inputs of a broad band n-th wavelength optical multiplexer, MUX, whose output is connected to a relevant n-th output fiber, on which a WDM signal consisting of M wavelengths xcex is transmitted.
From the above, we see that the architecture can be divided into N modules, as the one indicated as MODULK in FIG. 7, each one associated to an output fiber. Any OS, inside a particular module, works under thy constraint of transmitting a determined and fixed wavelength xcex, imposed by the particular multiplexer (MUX) port to which it is connected; this is the only way to obtain a correct WDM signal. The signals at the output of tunable filters correspond to a same number of CHk,l channels, indicated with two indexes, out of which, the first one (k) is referred to the n-th module and the second one (l) to the m-th wavelength; it is worth to recall that the physical wavelength by which the signal is carried, before an OR, can be different from the one transmitted by the corresponding OS, without constraints, since the OR/OS pair, making a double O/E/O signal conversion, can implicitly act as a wavelength converter (as said above). It is helpful to clarify that the cascade of the two input splitters, accomplishing two subsequent subdivisions of the signal by 1:N and 1:M, can be also seen as a unique subdivision of the same signal by one splitter 1:(Nxc3x97M).
The remarkable operational parallelism of the input stage, which carries the WDM signals from the N input fibers up to the, Nxc3x97M single channels OR/OS regenerators, can be noticed. Thanks to this configuration, it is possible to extend up to the input of the M channel filters, of all the N MODULK modules, any of the entering optical fibers; the simultaneous connection of several fibers to a same filter being prevented by optical selectors placed upstream the filters. It is instead possible to extend a same entering fiber to all the N modules and implement the broadcasting of the concerned WDM signal, or of a single xcex towards all the output fibers. To this purpose, it is sufficient to arrange the optical selectors such as to choose the signal from the same entering fiber, in all the MODULK modules, and to set tunable filters according to the same selectivity scheme for all the MODULK modules.
Though with the advantages highlighted in the input stage, the architecture of FIG. 7 has the disadvantage of being not as well effective in the output stage, in practice, reduced to the sole N multiplexers MUX. This constraint, as we shall see, negatively reflects on the cross-connect to the extent to nullify the potential advantages due to the high parallelism of the input stage. Also, a clear indication of how to perform the drop/insert of local channels is missing.
A common constraint, for all the architectures described up to now, and those shown in the mentioned technical literature, is that of being not sufficiently flexible as for the protection of local channels for which the drop/insert functionality is required. In fact, it can be noticed that in the known cross-connects the protection is made on the whole WDM aggregate of a fiber, therefore, at least in the case of architectures similar to those of the previous two patent quotations, it is necessary to foresee specific protection schemes for local channels, separate from those foreseen for transit channels. On the contrary, in the case of the cross-connector of FIG. 7, in lack of clear references drawn out from the article, we thought, just to demonstrate the conclusions that will follow, to produce ex-post facto, part of the novel technical characteristics proper of the invention which must still be described. That is, we assumed to modify the blocks of OR/OS regenerators to introduce the drop/insert functionality. Now, it is immediate to notice from the simple analysis of FIG. 7 that this modification, in case of break of an output optical fiber, would completely nullify the possibility to protect channels locally inserted in a MODULK module, referred to the broken k-th fiber. The trouble is due to the absence, of flexibility in the output stage.
Therefore, object of the present-invention is to overcome the drawbacks of the cross-connects of the known art, such as for instance the last highlighted disadvantage, or the stringent requirement of a single non-blocking spatial matrix, or the need to increase the size of the matrix, or of star-couplers, to accommodate the additional optoelectronic modules of locally drop/insert channels.
A further object of the invention is to indicate an optical cross connect architecture having improved flexibility of operation and scalability.
A further object of the invention is to indicate an optical cross-connect architecture that supports flexible protection schemes, and other requirements to tailor the node characteristics to different contexts it might operate into.
To attain said objects, scope of the present invention is an optical channel cross-connect connected to N optical input fibers and at least a same number of output fibers, the same being crossed by relevant signals obtained from the multiplexing of M component flows transmitted at different wavelengths, and each n-th entering fiber being connected to an Nxc3x97M ways n-th input optical splitter, said Nxc3x97M ways being connected to n-th inputs of a set of Nxc3x97M input optical selectors having N inputs and one output, the outputs of said optical selectors being connected to relevant optical channel filters tunable to any of the M wavelengths of the input signal, said filters being subdivided to form N groups of M filters and each group of filters sending the M filtered channels towards an output stage including N optical combiners having N inputs and one output, the N outputs of the optical combiners being connected to a same number of output optical fibers on which M channels are multiplexed, wherein the output stage additionally includes:
Nxc3x97M bridge units, whose functionality is selectable on individual basis, each unit being connected downstream a relevant channel filter to receive one channel, at any wavelength, on which its functionality is performed;
Nxc3x97M output optical splitters having N output ways, each output splitter being connected downstream a relevant bridge unit to receive an optical signal at the wavelength on which the above functionality is performed, making it available on N ways;
Nxc3x97M output optical selectors having N input and one output, placed downstream said optical splitters to receive optical channels on N ways, and select one to be sent to an input of said optical combiner included in the output stage; the connections among said optical output splitters and selectors providing an m-th channel coming from one said n-th group of channel filters, at an n-th input of output optical selectors connected to relevant m-th inputs of said optical combiners, as described in claim 1.
The architecture of the cross-connect scope of the present invention can simultaneously achieve all the above mentioned objects, thanks to a xe2x80x9csplit and selectxe2x80x9d stage, increasing the routing opportunities of a channel towards all the output fibers and to a bridge stage, which can be configured at single channel level, placed across an input stage of the xe2x80x9cparallel xcex switchxe2x80x9d type and of the above mentioned split and select output stage. The architecture is strictly non-blocking, both in the input stage and in the output one, separately considered. The set of channel filters, which can be tuned on the whole used bandwidth, enables to face different formats of the WDM signals, in terms of number of channels and relevant spacing, without the need to employ fixed optical multiplexers/demultiplexers to routing purposes.
Thanks to the proposed architecture, it is now possible to locally insert any channel, with a fraction of locally inserted to throughput channels ranging from 0 to 100%, without equipping for this reason dedicated modules expanding the system size. To this purpose, it is sufficient to enable the insert function in the bridge unit selected for the channel to be inserted; the same applies to the local extraction of a channel through the drop function. It should be appreciated that the splitting and selection functions, before the output combining, enable to re-route a single channel irrespective of the WDM aggregate to which the same is normally assigned; this implies that the protection of the single locally inserted channel is allowed, fulfilling the important customer requirements for flexible protection schemes. This protection mechanism co-exists with the scheme for the protection of transit channels that foresees to switch on a stand-by fiber the whole WDM aggregate of an interrupted fiber, since this aggregate includes also channels locally inserted. In the description of the invention implementation below, the protection approach shall be resumed and expanded.
The idea to use functionally configurable bridge units, one for each of the channels that can pass through the cross-connect; allows obviously performing, if required, the regeneration of the monochromatic optical flows supporting the above- mentioned channels. The same applies to additional functionalities which may derive from system, specification, such as for instance the wavelength conversion, the possibility to process client signals in the electronic domain, in terms of overhead monitoring (e.g., on SDH frames), of exploitation of the unused portion of the client overhead to transport data necessary to the optical network itself, etc. On the other hand, whenever the transparency requirement (in terms of pure optical processing and independence on the client signal type) is imposed by network design constraints and permitted by the transmission characteristics of the network, the bridge units can be equipped with optical components capable to restore the transmission qualities of the signal in a merely optical way (amplification, equalization and power control).
From the above, it comes out the great flexibility made possible by the bridge units used in the cross-connect scope of the invention; said flexibility is achieved through the design of bridge units such to render the node suitable to operate in different contexts, and equipping the cross-connect with different combinations of the same.